1. Field of the Invention
The present invention relates to a metal mold for molding a honeycomb structure that is used as a catalyst carrier or the like in, for example, a device for cleaning the exhaust gas from an automobile, and to a method of producing the metal mold.
2. Description of the Related Art
A ceramic honeycomb structure comprising, for example, cordierite as a chief component is produced by extrusion-molding a material by using a metal mold. The honeycomb structure constitutes a number of cells by forming the partitioning walls in the form of a lattice, and the cells assume, for example, a hexagonal shape.
To produce a honeycomb structure having cells of the hexagonal shape (hereinafter referred to as hexagonal honeycomb), a metal mold having slit grooves of the shape of a hexagonal lattice must be used and the partitioning walls must be formed in the shape of a hexagonal lattice.
A conventional metal mold for producing a hexagonal honeycomb structure has, as shown in FIGS. 1A and 1B, feed holes 11 for feeding a material and slit grooves 3 formed in the shape of a hexagonal lattice and communicated with the feed holes 11.
To produce this metal mold 1, the feed holes 11 are formed by drilling from one surface of the metal mold blank, and the slit grooves are formed in the shape of a hexagonal lattice from the other surface thereof by such machining means as electric discharge machining. Then, as shown in FIG. 1, the intersecting points of the slit grooves of the shape of a hexagonal lattice are communicated with the feed holes 11 to thereby obtain the metal mold 1.
However, the conventional metal mold 1 for producing the hexagonal honeycomb structure has problems as described below.
That is, in order to uniformly form the partitioning walls of the hexagonal honeycomb structure by using the above-mentioned conventional metal mold 1, the depth of the slit grooves of the shape of a hexagonal lattice must be selected to be not smaller than 10 times as great as the width of the grooves. Therefore, an extended period of time is required for forming the slit grooves.
Furthermore, when it is attempted to form the slit grooves relying upon, for example, the electric discharge machining, the electrodes are worn out during the machining often causing a dispersion in the depth of the slit grooves. In this case, therefore, the partitioning walls of the obtained hexagonal honeycomb structure loses uniformity.
To produce the metal mold 1, furthermore, a metal mold blank 4 is prepared having a hole-forming surface 41 in which the feed holes 11 will be formed and having a groove-forming surface 43 in which the slit grooves 3 will be formed (see FIG. 14). The feed holes 11 are formed by drilling in the hole-forming surface, the slit grooves 3 of the shape of a hexagonal lattice are formed by the electric discharge machining in the groove-forming surface, and the slit grooves 3 and the feed holes 11 are communicated with each other thereby to obtain the metal mold 1.
Referring to FIG. 2, the electric discharge machining is carried out by using an electrode 81 for the electric discharge machining provided with a working surface 80 of the shape of a lattice corresponding to the whole surfaces of the slit grooves 3 that are to be formed, and repeating the electric discharge between the electrode 81 for the electric discharge machining and the groove-forming surface 43 of the metal mold blank 4 in a working solution. The working solution is fed from a working solution-feeding pipe 95 of a working solution-feeding jig 9 disposed on the back surface side of the electrode 81 for the electric discharge machining.
However, the above-mentioned conventional method of producing the metal mold for forming a honeycomb structure has problems as described below.
That is, the slit grooves 3 have heretofore been formed by the electric discharge machining by using an electrode for the electric discharge machining having the shape of a lattice corresponding to the whole slit grooves that are to be formed. During the electric discharge machining, the electrode for the electric discharge machining is often distorted or worn out in varying amounts and is deformed. In such a case, the depth of the slit grooves varies causing a problem from the standpoint of quality.
On the other hand, the electrode for the electric discharge machining is made of a very hard material such as a tungsten alloy or the like, and is produced requiring a long period of time of, for example, several tens of days. When it is attempted to newly produce a metal mold for molding a honeycomb structure, therefore, several tens of days are, first, required for producing the electrode for the electric discharge machining and, then, another several tens of days are required for forming the slit grooves by the electric discharge machining, which is a very long lead time.
The present invention was accomplished in view of the above-mentioned problems inherent in the prior art, and its object is to provide a metal mold for molding a honeycomb structure, capable of precisely and efficiently forming the slit grooves within a short lead time and exhibiting good moldability, and a method of producing the same.
A first invention is concerned with a metal mold for molding a hexagonal honeycomb structure, having feed holes for feeding a material, pool grooves formed in the shape of a triangular lattice and communicated with the feed holes, and slit grooves formed in the shape of a hexagonal lattice and communicated with the pool grooves.
In this invention, the most important point is that the pool grooves of the shape of a triangular lattice are formed between the feed holes and the slit grooves.
The pool grooves are formed in the shape of a triangular lattice by, for example, regularly and alternatingly arranging equilateral triangles in the opposing directions.
It is further desired that the pool grooves and the feed holes are communicated with each other at the intersecting points of the triangular lattices of the pool grooves. This permits the material to smoothly flow from the feed holes to the pool grooves. In this case, the feed holes need not necessarily be communicated at every intersecting point of the pool grooves, but many be constituted in various ways by taking into consideration the size of the honeycomb structure that is to be molded and the moldability. For example, the feed holes may be communicated with every second intersecting point or with every third intersecting point.
It is desired that each hexagonal lattice of the slit grooves is so formed as to come into agreement with a hexagon shaped by combining six triangular lattices of the pool grooves.
In this case, it is possible to more uniformly and smoothly move the material during the extrusion molding.
Here, the hexagon shaped by combining six triangular lattices of the pool groups stands for the one formed as an outer shape of when six triangles are viewed as a unit, the six triangles being radially arranged neighboring each other about an intersecting point of the pool grooves.
In this case, therefore, when the slit grooves and the pool grooves are viewed from the front, the pool grooves are located at portions overlapped on the hexagonal slit grooves and on the boundary portions of the six triangles formed by connecting the vertexes thereof and the centers thereof.
A second invention is concerned with a method of producing a metal mold for molding a hexagonal honeycomb structure, having feed holes for feeding a material, pool grooves formed in the shape of a triangular lattice and communicated with the feed holes, and slit grooves formed in the shape of a hexagonal lattice and communicated with the pool grooves, each hexagonal lattice of the slit grooves being so formed as to come into agreement with a hexagon shaped by combining six triangular lattices of the pool grooves;
wherein a metal mold base for forming the feed holes, and a groove-forming member (metal mold blank) having a pool groove-forming surface and a slit groove-forming surface, are prepared;
said feed holes are formed in said metal mold base so as to penetrate therethrough, and a plurality of pool grooves intersecting at an angle of about 60 degrees relative to each other are formed in the shape of a triangular lattice in said pool groove-forming surface of said groove-forming member;
said pool groove-forming surface of said groove-forming member is joined to said metal mold base; and
said slit grooves of the shape of a hexagonal lattice are formed in said slit groove-forming surface of said groove-forming member so as to be communicated with said pool grooves.
In this invention, the most important point is that the pool grooves are formed in the shape of a triangular lattice in the pool groove-forming surface of the groove-forming member (metal mold blank), the pool groove-forming surface of the groove-forming member is joined to the metal mold base provided with the feed holes and, then, the slit grooves of the shape of a hexagonal lattice are formed in the slit groove-forming surface of the groove-forming member.
The feed holes are formed in the metal mold base by various machining methods such as drilling, electric discharge machining or the like.
Furthermore, the pool grooves are formed in the groove-forming member relying upon such a method that the operations for forming a plurality of straight grooves in parallel are executed from the three directions to intersect at an angle of about 60 degrees. In this case, the straight pool grooves can be efficiently formed by cutting or grinding by using a rotary tool that features a high working speed.
The slit grooves are formed in the groove-forming member after the groove-forming member and the metal mold base have been joined together. The junction in this case is accomplished by a variety of methods such as diffusion bonding, welding, adhesion with an adhesive, etc.
Since the slit grooves are formed after the junction, it is allowed to prevent the groove-forming member from being split at the time when the slit grooves and the pool grooves are communicated with each other.
The slit grooves can be formed by any machining method such as electric discharge machining, cutting or laser beam machining. Since the depth of the slit grooves can be smaller than that of the prior art, various machining methods can be employed without being affected by the wear of the tools.
Here, the electric discharge machining is a machining method which is based on the electric discharge between an electrode and a workpiece as is well known. The cutting can be accomplished by using a rod-like cutting tool having a cutting side surface and by moving the cutting tool while rotating it. The laser beam machining is a machining method which is carried out by irradiating the working surface with a laser beam.
A third invention is concerned with a method of producing a metal mold for molding a hexagonal honeycomb structure, having feed holes for feeding a material, pool grooves formed in the shape of a triangular lattice and communicated with the feed holes, and slit grooves formed in the shape of a hexagonal lattice and communicated with the pool grooves, each hexagonal lattice of the slit grooves being so formed as to come into agreement with a hexagon shaped by combining six triangular lattices of the pool grooves;
wherein a metal mold blank having a feed hole-forming surface and a slit groove-forming surface is prepared;
feed holes of a predetermined depth are formed in said feed hole-forming surface of said metal mold blank; and
a plurality of pool grooves intersecting at an angle of about 60 degrees relative to each other are formed in the shape of a triangular lattice in said pool groove-forming surface of said metal mold blank, and the pool grooves, except those of the hexagonal lattice portion where said slit grooves are to be arranged, are closed thereby to form said slit grooves.
In this invention, the most important point is that the pool grooves and the slit grooves are formed in a manner that the pool grooves of the shape of a triangular lattice are formed first and, then, some of the pool grooves are closed to form the slit grooves. Here, the closure may be effected by stuffing the interior of the pool grooves with a closing agent or by covering the opening portions of the pool grooves.
The feed holes can be formed in the metal mold blank by various machining methods such as drilling, electric discharge machining, etc. The depth of the feed holes is so selected as can be communicated with the pool grooves. Here, the feed holes may be formed before or after the pool grooves or the slit grooves are formed.
The pool grooves can be formed in the slit groove-forming surface relying upon such a method that the operations for forming a plurality of straight grooves in parallel are executed from the three directions to intersect at an angle of about 60 degrees. Here, the depth of the pool grooves is the sum of the depth of the slit grooves that are to be formed and the depth of the pool grooves.
In order to form the slit grooves, the pool grooves are closed by various methods as will be described later. The closure in this case is accomplished to exhibit a strength large enough to withstand the pushing pressure at the time when the extrusion molding is practically conducted by using the metal mold for molding a hexagonal honeycomb structure.
It is desired that the pool grooves of the shape of a triangular lattice according to the third invention are formed by cutting or grinding. This makes it possible to very efficiently form the pool grooves. The working tool in this case will be a rotary tool such as a circular thin-bladed grind stone.
The pool grooves can be closed by laser beam welding. In this case, the positions of the closing portions can be easily determined by controlling the irradiation pattern of the laser beam, to execute the closing processing maintaining a high precision. The laser beam welding can be conducted by either a method by which the opening portions are closed by melt-adhering both walls of the pool grooves that are to be closed or a method by which the opening portions are closed by welding another member such as a welding rod.
Furthermore, the pool grooves are closed by, first, stuffing the whole pool grooves of the shape of a triangular lattice with a closing agent, permitting the closing agent to be selectively coagulated in the pool grooves except those of the hexagonal lattice portion where said slit grooves are to be arranged, and removing the uncoagulated closing agent from the slit groove portions. In this case, the closing depth of the closing portions is adjusted depending upon the amount of the closing agent. Therefore, the depth of the pool grooves can be easily adjusted.
A metal powder or a thermosetting resin is used as the closing agent, and the closing agent is selectively coagulated upon solidifying or sintering by being irradiated with a laser beam. In this case, too, the positions of the closing portions can be easily determined by controlling the irradiation pattern of the laser beam, to execute the closing processing maintaining a high precision.
Furthermore, a photocuring resin can be used as the closing agent, and the closing agent is selectively coagulated by the irradiation with light in a state where the slit groove-forming portion is masked. In this case, heat is not generated in large amounts during the closing processing, and the metal mold is reliably prevented from being affected by heat.
Moreover, the pool grooves are closed by, first, stuffing the whole pool grooves of the shape of a triangular lattice with a false closing agent, permitting the false closing agent to be selectively coagulated in the pool grooves in the hexagonal lattice portion where said slit grooves are to be arranged, removing the uncoagulated false closing agent from the slit groove portions, closing the pool grooves from which said false closing agent is removed with a closing agent, and removing the false closing agent from said slit groove-forming portion.
It is desired that the closing agent is a plated layer. This makes it possible to easily accomplish the closing processing. In this case, it is desired to use the false closing agent which exhibits the effect for preventing the formation of the plated layer. After the plating, therefore, the false closing agent can be easily removed.
A fourth invention is concerned with a method of producing a metal mold for molding a honeycomb structure, having a plurality of feed holes for feeding a material and slit grooves formed in the shape of a lattice being communicated with said feed holes to mold the material into a honeycomb shape, wherein the slit grooves are formed in the groove-forming surface of the metal mold blank by the electric discharge machining that is executed a plural number of times by using an electrode for the electric discharge machining having a working surface of an area smaller than the area of said groove-forming surface.
In this invention, the most important point is that the slit grooves are formed by the electric discharge machining that is executed a plural number of times by using an electrode for the electric discharge machining having a working surface of an area smaller than the area of the groove-forming surface of the metal mold blank.
As described above, the electrode for the electric discharge machining has a working surface of an area smaller than that of the groove-forming surface, and is smaller than the conventional electrode for the electric discharge machining.
The electric discharge machining may be executed a plural number of times repetitively by using the above-mentioned small electrode for the electric discharge machining or by using another small electrode for the electric discharge machining after each time or after a plurality of times.
According to the fourth invention, it is desired that the working surface of the electrode for the electric discharge machining is of a size capable of machining one region among n regions of said groove-forming surface that is divided into n regions in the direction of width, and the electric discharge machining is executed by repeating, a plural number of times, a unit work which works said n regions to accomplish a predetermined depth by using one or a plurality of electrodes for the electric discharge machining.
That is, the regions are not worked to a predetermined depth through one time of the electric discharge machining but, instead, the whole groove-forming surface is worked to a predetermined depth through the above-mentioned unit work, and the unit work is repeated to increase the depth of the grooves. Thus, the electric discharge machining is effected being divided into a plurality of times not only in the direction of width but also in the direction of depth, suppressing local variance in the machining and enhancing precision for machining the slit grooves.
It is desired that the unit work is carried out in a manner that the central region located nearly at the center is electrically discharge-machined, first, among the n regions and, then, the regions are successively machined to separate away from the central region. In this case, changes in the width of the slit grooves due to very small variance in the machining can be set to be symmetrical in the right-and-left direction. This makes it possible to improve the moldability at the time of molding the honeycomb structure by using the obtained metal mold for molding a honeycomb structure.
It is desired that in the working surface of the electrode for the electric discharge machining, every portion that contributes to the machining has the shape of a lattice corresponding to the lattice shape of the slit grooves, and has no incomplete side that does not form the lattice. In this case, it is possible to improve the machining precision at the boundaries of the neighboring electric discharge-machining portions.
It is further desired that among the plural times of the electric discharge machinings, the second and subsequent electric discharge machinings are executed by so moving the electrode for the electric discharge machining that at least one of the lattices of the working surface is overlapped on the lattice formed by the preceding electric discharge machining. In this case, it is made possible to prevent deviations in positions of the lattices of the formed slit grooves.
It is further desired that the electrode for the electric discharge machining is provided with a working solution-feeding jig for feeding a working solution for discharge working, and said working solution-feeding jig has two or more working solution injection ports. In this case, the working solution is uniformly fed onto the working surface to remove the sludge and, hence, to uniformalize the electric discharge. Therefore, this contributes to further improving the precision for forming the slit grooves.
The present invention will be more fully understood from the description of preferred embodiments of the invention set forth below together with the accompanying drawings.